"We barely spend 2% now and that is utterly inadequate. Therefore if Gavin Williamson is prepared to have a bare-knuckle fight with the Treasury to get defence spending raised to the level it absolutely needs, and if he wins that fight, he will deserve our total support."

His comments come amid a reported rift between Mr Williamson and Philip Hammond after recent swipes between the pair.

Yesterday the Chancellor denied he was cutting cash sent to the armed forces, and suggested Mr Williamson – who took over from Michael Fallon last month – had yet to fully get to grips with the role.

He told the Treasury Committee: “I expect that once he has had a chance to understand the situation in the Ministry of Defence and to get his head around the defence budget, the new Defence Secretary will be wanting to come and talk to me and he will find no one more sympathetic to the challenges of defence than me.”

And his quote followed yesterday’s Times front page, which said the RAF would stop flying the Chancellor until he has paid for previous ministerial flights – a claim which is said to have left Mr Hammond “furious”.

"It was a cheap shot, childish and completely unhelpful," an ally of the Chancellor told Sky News. "Why would Phillip want to help out Gavin after that?"

A meeting between the two ministers was reportedly scheduled for Monday, but was cancelled at short notice and a new date has yet to be scheduled.